Deletion of the Selenocysteine tRNA Gene in Macrophages and Liver Results in Compensatory Gene Induction of Cytoprotective Enzymes by Nrf2

Abstract

The selenocysteine tRNA (tRNA Sec ) molecule is the sight of synthesis for the amino acid selenocysteine and the adaptor for its translational insertion into selenoprotein enzymes, the majority of which contribute to cellular redox homeostasis. To examine the consequences of selenoprotein depletion on the oxidative environment of the cell, we generated a conditional knock-out mouse for the tRNA Sec gene (Trsp). Deletion of Trsp in either macrophages or liver elevated oxidative stress and activated the transcriptional induction of cytoprotective antioxidant and detoxification enzyme genes, including glutathione S-transferase P1 and NAD(P)H:quinone oxidoreductase 1, and other well known target genes of the transcription factor Nrf2 (NF-E2-related factor 2). Simultaneous disruption of Trsp and Nrf2 severely compromised the cytoprotective response. Double knock-out macrophages displayed reduced viability, elevated oxidative stress, and increased susceptible to hydrogen peroxide treatment compared with deletion of either gene alone. Mice carrying a liver-specific deletion of Trsp on an Nrf2-null background experienced hepatocellular apoptosis and displayed a severely reduced survival rate compared with loss of Trsp alone. Our results thus demonstrate that reduced selenoprotein activity is counterbalanced by an Nrf2-mediated cytoprotective response, which is essential for maintaining cellular redox homeostasis and viability.

Nrf2 target genes are induced under marginal selenium-deficiency

“…Transactivation of target genes is induced in response to reactive oxygen species (ROS) or electrophilic agents that modify susceptible thiol groups of Keap1, resulting in a rapid increase in nuclear Nrf2 protein levels (reviewed in [38]). A simultaneous disruption of Trsp and Nrf2 abolished the induction of phase II enzymes, thus, verifying the role of Nrf2 in responding to the loss of selenoproteins [52]. The ARE-driven reporter gene activity in ARE-reporter mice fed a selenium-deficient diet was strongly enhanced compared to mice fed the control diet [9].…”

“…High concentrations of certain selenium compounds or metabolites may directly activate the Nrf2 pathway by modifying critical thiols in Keap1. Reactive thiol groups can also be modified by oxidation [22,29] a situation which prevails in selenium-deficiency [52,55]. Whether a higher oxidative status in the moderately selenium-deficient mice was responsible for the induced Nrf2 target gene expression needs to be clarified.…”

“…The idea of a compensatory up-regulation of antioxidant and phase II enzymes in selenium-deficiency was further supported by findings in mice with an organ-specific targeted removal of the gene encoding for selenocysteine tRNA (Trsp). The complete loss of selenoproteins in liver resulted in the induction of GSTPi, NQO1 and HMOX1 [52].…”

Basic Principles and Emerging Concepts in the Redox Control of Transcription Factors

“…The unexpected phenomenon could not be convincingly explained by GPx1 deficiency alone (396). However, at least one of the selenoproteins had to be involved in Nrf2 activation, since a complete loss of all selenoproteins by an organ-specific removal of the gene encoding selenocysteine tRNA (Trsp) (56) resulted in the induction of GST isoforms in liver, of GSTP1 and NADPH quinone oxidoreductase 1 (NQO1) in liver and macrophages (471) and of heme oxygenase-1 (HO-1) in liver (446). Also a moderate selenium deficiency in which only selenoprotein W, GPx1, and selenoprotein H and M were markedly decreased (249) led to an increase in NQO1, GSTs, sulfotransferases, and UDP-glucuronyl transferases (UGT) as well as HO-1, Prx1, sulfiredoxin-1 (Srx1), and c-glutamylcysteine synthase (335).…”

Selective up-regulation of human selenoproteins in response to oxidative stress

“…The degree of expression of these genes involves other transcription factors (30,31). A link between selenium, selenoprotein expression, and the antioxidant response has been established via NRF2, which is induced in certain cell lines during severe selenium deficiency or complete loss of selenoproteins obtained by tRNA Sec gene inactivation (32). The remarkable feature of selenoprotein expression lies in the many levels of control.…”